text:
Can a helicopter really fly with rotors going this slow?
I was watching this youtube video and I noticed that at 3:17 seconds they show a coast guard helicopter rescuing a man from a ship. Whats shocking about this is that the main rotor of the helicopter seem to be going incredibly slowly and the tail rotor has completely stopped.
How was the helicopter able to stay flying with such a low rotational speed? Was this some optical illusion or was the rotor really spinning that slowly?
Also, how was the helicopter not spinning? From what I understand the main purpose of the rear rotor is to counteract the rotational force of the main rotor. Since the rear rotor wasn't moving, shouldn't the helicopter developed a spin counter to the spin of the main rotor (even though it was slow)?

It's almost certainly just a frame-rate effect. The rotors won't actually be spinning that slowly.

@DavidRicherby Thats what I thoguht at first too, but if you look at the man and the water the framerate looks fine.

I meant that that it's the usual thing where, because you're looking at a sequence of still images shot at a particular frequency, things that are rotating quickly often seem to be moving slowly or even backwards. You can often see this in videos of car wheels. For example, it's impossible to tell the difference between the blades rotating 10 degrees per frame and 100 (=90+10; it's a four-rotor machine) degrees per frame or even 190 degrees per frame, because the resulting sequence of still photographs would look identical.

@DavidRicherby but usually when that happens there is a bit of a blink and other imperfections. I've never seen it so smooth. Also the trail rotor is perfectly still, I've never seen a frame rate effect like that.

It's a simple optical illusion. A common, and well known strobe effect. Nothing more, nothing less.

Here's another good example, and another.  In the second one, the blades appear to go backwards.

The blades are blurred due to their speed. If the speed was slow, they would have very sharp edges, like the cabin. This is not possible to fly with so few air blown downward per second -- could be a good speed for a ceiling fan though :-) The tail rotor blades are not visible, you see the fixed struts maintaining the rotor axis.

The tail rotor doesn't appear to be stopped. you can't see the blades at all; they are just a blur. What you are looking at are the flow-straigtening stators in the fenestron tail. They don't move

By the way, that's not a rescue, they are simply picking up a pilot (the naval kind).

@TomMcW: Ah I thought it was simple struts! BTW it's possible to see marks on the blades of the anti-torque on this contrasted snapshot. And yes as Jörg said it's an helicopter used to carry pilots on/from ships in the Bordeaux area (Google translation).

Please do note that the link to the video is broken.

It's an illusion that the blades appear to be going slowly. It's actually a well known effect called the wagon wheel effect.
Essentially the rotor is spinning at close to an even multiple of the camera's framerate divided by the number of rotors.
This means that between frames the blades have moved a full quarter rotation (or a multiple of that). Creating the illusion that the rotor is moving more slowly that it actually is.

You could add the point about the fenestron brought up by @TomMcW in the comments above. To quote part of his comment here: "you can't see the blades at all; they are just a blur. What you are looking at are the flow-straigtening stators in the fenestron tail."

"This means that between frames the blades have moved a full quarter rotation": Or any number of quarters (if there are 4 blades on the rotor).

The shutter effect can even make the rotor to appear to not turn at all

Life is full of illusions and many things give similar illusions. Strobe lights are popular with dance and and other entertainment venues. One game was to adjust the strobe timing so a spinning wheel would look like it was going backwards, then forwards, then standing still, and so on until you grew tired of the game.
In old movie westerns wagon wheel spokes sometimes look like they are going backwards.
In the days of old CRT monitors TV shows had to use special electronics with TVs filmed on show sets to avoid the video appearing to break up or doing other funny things.
You can get a simple strobe effect with many modern LED Christmas light strings. Move your eyes rapidly so your vision crosses the lights and you may well see the lights appear to flash. Actually, they are always flashing. It is just that they are flashing faster than the eye can normally notice. If you don't want to get dizzy, you can shake a handheld mirror to see the same effect.
Edit: 2023-Feb-06: more and more I'm noticing wheels on cars, etc., on online videos, with there lower refresh rates and higher compressions, can get very noticeable with how uneven they jerk and spin.

And they were also used to adjust ignition timing on cars, when ignition was still mechanical. Also on turntables. Turntables are not well known yet, but are top hype :-)

Also very common-- rear-view mirror in car vibrates slightly so you can get a strobe effect-- if a vehicle behind you has exposed tires with large treads (e.g. tractor, road grader, high-clearance truck with oversized tires), they may appear to be stationary or slowly rotating backwards --

That's an illusion, the result of the frame rate of the video camera being a multiple of the rotor speed. It so happens that when the video takes an individual frame, the rotor blades are at or near the same position, so the rotor appears to be turning very slowly or in some cases, to be stationary. If you look up recent videos of large prop planes being started, you can often see the prop appear to become stationary (the prop blades also appear to be bending) as they rev the engine up... same basic effect. You don't see this in old films, because the film was of such a slow speed with a long exposure time for each frame that a moving rotor would be blurred.
However... there was one helicopter whose main rotor ran at a very leisurely 88 rpm... the Hughes XH-17. It was cobbled together from a variety of sources, including a cockpit from a Waco CG4 glider, to test the feasibility of a very large helicopter with rotors driven by jets at the tip of the rotor blades. Fuel was sprayed into the tip jets and ignited to provide rotor propulsion, hence the very small tail rotor - very little torque was produced by this system.
The two turbine engines are there only to supply compressed air to the jets at the ends of the rotors. The XH17 has to date, the largest rotor blades ever fitted to a helicopter, with a 129 foot main rotor diameter.


text:
Is this one of the engines from the 9/11 aircraft?
I ran across an interesting set of post-9/11 photos and I noticed this picture in particular:

The closest item on the truck looks a bit like an airplane engine, but I can't tell for sure. Is it one of the engines from the 9/11 aircraft?

They are definitely not jet engines. They look more like electric motors or generators or blower units or something.

yeah, that was my impression too. Way too solidly constructed, what OP probably thinks are fan blades aren't, they look to be non-moving structural parts.

I'm guessing that those are actually elevator motors. In fact Google Image Search provides some very similar results and apparently one exists that you can go see at the memorial museum.

The windings are a dead giveaway that the one on the right is a rotor from an electric motor/generator (which are essentially the same thing used differently). Also, the "fan blades" are actually fan blades, but they rotate together with the rest of the rotor. They're there to force air through the rotor to cool the windings.

Also, jet engine cores don't have cylindrical casings. The diameter changes along the length.

These are very large electric motors, used in HVAC plants or water distribution, probably. Or possibly main generators out of a diesel gen-set.
They are very dense and tough by design, though certainly beat to snot; they may have been inside a building that collapsed.  They are on the trailer together because they are going a scrapper who specializes in hacking copper out of motors. Copper is worth $4-8 a kilo depending on market. They use copper for such windings because copper has the best conductivity per volume, for more efficient use of the magnetic fields.
Such a thing is far too heavy and dense to be any part of any airplane. Generally speaking, copper's density works against it in aviation applications; aluminum delivers the same conductivity at almost half the weight (although at nearly double the volume, but volume is not as important as weight).

The density of interesting facts in this answer is exceptional.

"They may have been inside a building that collapsed": they were inside one of the World Trade Center towers when it collapsed.  The building in the background is the former New York Telephone Company Building, which occupies the block bounded by West St., Barclay St., Washington St., and Vesey St.

Those are Elevator shaft motors, they're wound for high starting torque, not like most pump motors.
Here is one of the motors from 2 other angles.  It's on exhibit at the 9/11 World Trade Center Memorial & Museum.
Photo 1 (Source "More Than Route 66" blog):

Next to the radio tower was an elevator shaft motor also recovered
  from the rubble. The motor was most likely from one of the express or
  service elevators from inside the north tower.

Photo 2 (Source Edward Stojakovic):

9/11 Memorial Museum - Elevator Motor

Unlike many of the others who have responded here, I have relatively little expertise in identifying motors and wouldn't be able to distinguish a pump motor from a high-torque lift motor at a glance.
I'm not even very expert when it comes to jet engines.
However, I do think that one reasonably reliable heuristic is this: if it appears to be attached to a large block of concrete, it is almost certainly not part of an airliner.
On that basis I think I can say with confidence that these are not jet engines.

This may well be the most convincing evidence of all! Thanks for pointing out the obvious that none of the rest of us saw.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Why did LaMia flight 2933 plan a flight longer than its listed fuel range?
According to the wiki, the range of the airplane used in the LMI 2933 flight has a range of 1600 miles. The distance between the airports though is 1605 miles though. I presume the exact range varies on conditions, but why would a flight that is outside the listed range be planned? This seems like an amateur mistake that should have been quickly caught on pre-flight checks.
Also, the wiki states that before crashing the plane made 2 loops. If the flight was running out of fuel, why would they do this? It seems like the flight could have easily made it if they flew directly, and considering that they had declared an emergency the ATC wouldn't be able to say no.

because the investigation is still going on and until it is done, we won't know.

Possible duplicate of When does the airline check if an airplane can reach its destination?

I'm voting to close this question as off-topic because it requests information from an ongoing investigation.

We avoid most questions on active investigations because they just end up in speculation, but you may want to look at the avherald page for this accident. It's a relatively well-informed and objective site, although it can be a bit technical. It answers your second question about the loops (holding pattern): a previous aircraft had already declared an emergency, so it got priority and 2933 was instructed to hold.

Have you heard about PANNIER tanks?

Only a journalist can answer this question...

The investigation is still on-going so I will stick to what is factually known. An airplane's range through the air should not be confused with travel over the ground, actual distance over the ground can vary significantly depending on the winds aloft. A significant tailwind will allow an airplane to travel farther than its listed maximum range, which is really just an estimate. We won't know what happened in this case until the investigation is complete.
According to the information about the flight the pilot did not declare an emergency, so was told to orbit while another airplane which had a mechanical problem landed, this explains the loops.

Why would you say that's speculation @KorvinStarmast?

I see your point @KorvinStarmast.

Well, I don't know anyone who would rather die than declare an emergency, my point was that airplane fuel gauges aren't exact. The pilots probably knew they were pretty low but maybe not how low. That is, as you pointed out, speculation

Downvote removed, per your amendment.  Aviation accident history has examples of people who would prefer not to declare an emergency ... some of whom died as a consequence.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What is the lifespan of commercial airframes (in general)?
You always hear that the Boeing 737 have been flying for so many years and the Airbus A310 for that many years, but of course that include new airframes being produced all the time. What is the lifespan of one airframe on average in general use by airlines, before it is scrapped?

more important than years is number of flights, the decompression cycles induce metal fatigue, air aloha flight 245 expired before it's time because it made mainly short haul flights.

Metal fatigue is normally the limiting factor of an aircraft's lifespan. Fatigue cracks build up every flight (at start & landing), called a cycle. Therefore the design life of an aircraft is typically given in cycles.

Metal fatigue cracks, source: Boeing

For long haul aircraft, that make relatively few cycles, the design lifespan is in the order of 40 000 cycles. For short haul the number is higher, sometimes up to 111 000 cycles.
For example, the Boeing 747 has a design number of cycles of 35 000, the MD-80 has 110 000.
An aircraft should be operable for about 30 years before reaching the cycle limit. The lifetime can be stretched by special inspection and maintenance programs.

It's worth noting that unpressurized aircraft can have similar limits for similar reasons - e.g. the Piper PA-38 Tomahawk has a wing life limit of 11,000 hours do to fatigue in the wing spar

It is however also worth noting that there are still some (unpressurized) aircraft manufactured 1940-1945 still in scheduled service.

@JanHudec Indeed - at that time there was no real thought of life-limiting parts due to fatigue, and aircraft certificated without life limits can basically keep flying essentially "forever" (subject to prudent inspections and maintenance of course). There are Cessna and Piper trainers that have no stated fatigue life which have over 10,000 hours on them, and with good maintenance (and barring any major accidents) they may fly another 10,000 hours before they're retired.

In which documents can such information be found( like design number of cycles?)

In many cases the decision to retire an airframe is economical rather than just "it's old". Proper maintenance can extend an airplane's service life almost indefinitely but it gets more expensive with age/flying time/cycles.
Also, new airplanes are cheaper to operate. If buying a new plane saves the purchase price in fuel in 5 years (quite possible) then you get one. It will not need any major maintenance in that period, further reducing the total cost of ownership. For example, a 707 and 737 are comparable in terms of capacity and range, but a brand-new 737 at \$90 million will cost a lot less over 5-10 years than a used 707 picked up for \$1 million.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Do helicopter rotor tips regularly go supersonic?
In my question about helicopter noise, I made the assumption that one reason they are so darn noisy is that the main rotor tips go faster than the speed of sound. @FreeMan questioned whether that was the case.
I've tried to find information about this but most pages end up discussing VNE -- the forward speed at which the advancing blades go supersonic while the retreating blades lose lift and stall. This is not about that situation.
This is about regular flight -- is the extremely loud noise from helicopters because their rotor tips are supersonic, or is from other sources?
p.s. There is some mention that the "WOP-WOP" of descending helicopters is caused by the rotor tips going supersonic. Is this the case - and if so, is it an edge case (the only time the tips are supersonic) or just one example of when they do.

The "wop wop", usually known as blade slap, is heard when the tip of the blade passes through the vortex created by the previous one.
It can be avoided.  The most common flight regime when this happens is a shallow descent but still with quite a lot of power - e.g. fast and shallow.  The vortex starts to move down as soon as it leaves the tip of the blade so in level flight, the following blade passes over it.  In a shallow descent with high pitch angle, the following blade can literally "slap" into the previous vortex.  The vortices of the two blades now interact and can cause local, transient supersonic flow.  To avoid it, simply lower the pitch to establish a more positive descent or pull back on the cyclic to increase disc loading and flatten the attitude.
The blade tips do not go supersonic.  In fact, in nearly all helicopter designs, the rotor rotates within a very narrow range of speeds, typically between 90% and 110% of the normal speed.  In most flight regimes, the rotor is rotating at 100%, +/- a few percent, whether you are climbing, descending or cruising.  Only during auto-rotation and aggressive manoeuvring does the range vary by 10% or more.  It depends on the type of helicopter but absolute limits would be something like 85% (panic time, risk of complete stall) and 115% (smaller panic, risk of damage to the machine, especially in the tail rotor drive shaft).
In normal operations, and design aims to achieve this, the rotor tips do not go supersonic since when they do, there is a sudden and large decrease in performance with more power required, higher blade loads, vibration and noise.
Think about a helicopter flying forwards.  The advancing blade at its most perpendicular position experiences a relative airflow which is equal (ignoring all kinds of minor side effects) to the forward speed plus the speed of the blade.  The retreating blade is experiencing a relative airflow equal to the speed of the blade minus the speed of the helicopter.
If the blades rotate so fast that the tips are supersonic, then the main lift generating part of the retreating blade, the outer two thirds of the span, would experience such a low airspeed, for some of the span it will even be negative, that the blades will stall causing a catastrophic roll into that side.  It is this phenomenon which ultimately limits the rotational speed of the blades and the maximum speed of the helicopter.
Let's look at the R22 as an example.  The following figures are approximate.
The rotor tip speed is about 670 fps (feet per second).  The speed of sound at ground level on a standard day is about 1100 fps.  The R22 is flying at close to VNE, let's say 100kts which is about 170 fps.
The tip on the advancing side, at it's fastest, is therefore flying relative to the airflow at 840 fps and on the retreating side, at it's slowest, at 500 fps.
The blade length is about 11 feet, so the middle portion of the blade on the retreating side is only flying at 190 fps (half of 670 minus the airspeed).  When you get to about 4 feet out from the blade root, it's now only 50 fps and not much further in from that, becomes zero, then negative.
Remember that lift is proportional to the square of the speed.  You can now see the huge discrepancy between lift on either side as airspeed increases.
To answer your question directly, the R22 would need to fly at 530 fps to approach supersonic tip speed which equates to about 330 kts which it can't get anywhere near achieving.
PS.  The R22 POH speaks in Imperial measures.  When I get some time, I'll redo the figures in metric which I, and most of the world, prefer.

The first sentence of your last paragraph seems to contradict itself: the disk rotates so fast the tips go supersonic, therefore the outer 2/3 of the span would experience slow airspeed. Could you please clarify how the tip (part of the outer 2/3 of the blade) can be supersonic, yet that same 2/3 of the blade can be moving too slowly? I'm not telling you you're wrong, I'm just really confused.

I will re-word it slightly.  I am talking about the retreating blade.  Thanks for pointing it out.

Is not the case that the forward moving rotor hitting the speed of sound is a limiting facter in the speed of the helicopter itself? I have a vague recollection of  a Lynx pilot telling me that when I was a kid.

@chriscowley It is, but you will hit VNE (velocity never exceed) first.  Read about dissymmetry of lift and how it relates to VNE

@FreeMan I found the answer confusing, too. I think it's saying this. The rotors spin fairly slowly so, when the helicopter is stationary, the tips are far from the speed of sound. The only way to get the tips to break the  sound barrier would be to move the helicopter forwards very quickly. So quickly, in fact, that the retreating blade would actually still be moving forwards, relative to the ground. That blade would would generate no lift because the airflow over it would be from what's supposed to be its trailing edge to the leading edge.

All of this assumes, of course, that the helicopter has only one main rotor.

... that the retreating blade would actually still be moving forwards, relative to the ground... And, more importantly, relative to the air.  : )

The characteristic beat of a helicopter rotor is caused by the interaction between the rotor blade vortices, in particular between the main rotor and the tail rotor vortices. As the shockwaves of these impulses coincide, they create powerful (loud) harmonics. This effect can occur at rotor speeds well below supersonic.
The vortex interaction can be reduced by surrounding (a smaller, multi-bladed) tail rotor - more like a fan - with a shroud. Such an installation is called a fenestron ("windowed", and actually a trademark belonging to Eurocopter), a ducted fan or fan-in-fin. This development was originally designed for improved safety and performance.
Modifications to the main rotor to reduce the impulse from the vortex typically trade away power or economy.
On the topic of supersonic speed, helicopters have a theoretical top speed of 417 kph in conventional flight mode because of the problem of the advancing blade reaching supersonic speed over too great an area and the retreating blade losing lift abruptly.
Someone asked how only part of the blade could be supersonic while the majority of its length was subsonic. This is because the movement is angular. A point on the outer portion moves through the air much quicker than a point on the inner region, to cover the same angle in the same time. This supersonic condition is reached sooner in flight than in hover. When the blade is moving "forward", the airspeed is added to the rotational speed of the forward moving blade and subtracted from the backward moving blade. A common solution to accommodate the difference in lift of opposing blades is to hinge them at the root to allow the blade with a higher airspeed to flap upwards to a limited extent. Some "rigid" designs replace the hinge with a flexible section.

Nice answer.  Welcome to aviation.se!


text:
How effective was WWII bombers self defense anti-air armament?
Bigger WWII bombers like e.g. the B-17 Flying Fortress were equipped with lots of machine guns for self-defense against single-seat fighter aircraft.
Wikipedia says:

Defensive armament increased from four 0.50 in (12.7 mm) machine guns
  and one 0.30 in (7.62 mm) nose machine gun in the B-17C, to thirteen
  0.50 in (12.7 mm) machine guns in the B-17G. But because the bombers could not maneuver when attacked by fighters, and needed to be flown
  straight and level during their final bomb run, individual aircraft
  struggled to fend off a direct attack.

The last sentence and the fact that fighter escort was developed and also personal impression from live footage etc. indicates that the self-defense armament wasn't very effective.
Is this true?
I can understand that bombers cannot do evasive maneuvers, but so does the attacking fighter plane when it is homing in on the bomber and aiming, isnt it?
In that moment the bomber actually should have the advantage of being able to point the (several) mobile guns, while the fighter must use the entire airframe to aim its guns and thus has a very predictable flight path.
And in case it really wasn't effective, why did they carry it then? Must have added a lot of additional weight.
Are there any figures to illustrate the effectiveness of bomber self defense?

There's a big difference between "struggled" and "totally failed." Moreover, live footage would tend to be biased.

The only figures I could find were the 60 bombers out of 300 lost by the USAAF in the Schweinfurt raid, most of those losses from German fighters. That convinced the generals that the onboard gunners weren't all that effective, and bombers couldn't defend themselves from a determined fighter attack. Major air attacks on Germany were postponed until the long range P51 could be deployed to protect the bombers better.
Gunnery from a bomber is far more difficult than from a fighter, that fires straight ahead. In a bomber, the gunner might be firing to the side, which means they have to factor in the bomber's speed that will propel the bullets sideways, and actually aim slightly behind the approaching fighter. The closer to the nose or tail, the less one aimed behind... bomber gunnery is very complex. This wartime training film illustrates the problems of firing on fighters approaching from the side. Try to calculate those ever changing angles when you're being shot at...
The same difficulties were experienced by ships being attacked by aircraft... very difficult to shoot down a small, fast moving aircraft approaching a ship. The Bismarck couldn't stop slow moving Swordfish biplanes from knocking it's rudders out. In the last major naval action of the war, eight Japanese warships (including the Yamato) were attacked by around 400 aircraft from US carriers. Seven of the eight ships were sunk, while only 14 aircraft were lost in the action, despite those ships having had quite a few extra AA guns added.
So, why arm the bombers? That was a question asked by Curtis LeMay in 1945. He devised a plan to take most of the guns out of B29's and operate them at night, because the Japanese hadn't demonstrated much in the way of night interception. He reduced the attack altitude to 12k feet from 30k feet, because that put the planes above small arms, but below the effective range of large anti-aircraft cannon. And, he used the weight saved from armament removal to carry extra incendiary bombs, to ignite the largely wood buildings of major Japanese cities.
The result was the fire raids, the most destructive bombing campaign of the war, even more destructive than the atomic bombs.
Guns on bombers have been dwindling since then, with the B52 having only one tail gun, while the B1 and B2 have no guns. Still, during Linebacker II, two MIG21's were shot down by B52 tail gunners, so the guns weren't completely useless.

Gunners do not "factor in the bombers speed". It is only relative Line of sight rate that affects the gunnery solution aim point. There is an effect referred to as :"Drag Shift" which requires that you aim in front of a target aircraft flying co-speed next to you, but this is not due to his airspeed, it is due to the deceleration of the bullet over it's time of flight. If you were firing a rocket, which accelerates over its time of flight, you would actually need to aim behind the target.

@CharlesBretana, the drag shift is not due to deceleration itself, but due to the relative wind accelerating it laterally in your frame of reference. This lateral acceleration applies equally to rockets, so even rockets must be aimed in front of a target to compensate for drag shift.

Apologies, @Jan Hudec, but you are wrong. If a projectile had just enough thrust to maintain velocity, you could aim directly at the target. ALL drag on the projectile moves it in the direction opposite to its relative wind. BY DEFINITION,  this drag (to whatever degree it exists), can only change projectile true air speed velocity parallel to its flight path. It CANNOT change velocity across its flight path. This is why, when bombing in a crosswind, aircraft must fly directly over the target for low drag bombs, upwind when dropping high-drag weapons, and downwind when firing rockets.

The "Training Film" mentioned in the answer is, in fact, also wrong. It is not the bombers forward velocity that causes the need top aim "behind" the attacking fighter. If you watch the film carefully, you will notice that the fighter is drifting backwards, towards the tail of the bomber in its attack profile. It is this rearward relative Line of sight rate that creates the need to aim towards the rear. The fighter is drifting rearwards because its current straight line flight path would carry it behind the bomber. The fighter is doing this so he can be turning forward when he fires.

If taught, and understood, properly, it is really not as hard as presented above, or, in the WWII training film. All you need to do is observe the relative line of sight (LOS) rate, or the apparent motion, of the target against the infinite backdrop. You can easily see that, even in the WWII training film. Then all you need to do is aim the gun in "front" of that motion, by an amount proportional to speed of that  LOS rate, and the time of flight (distance to TGT), and a little bit above for gravity drop.

@CharlesBretana, the drag can't indeed change velocity across its flight path in the reference frame of the free stream (well, if we were talking about ball anyway; gyroscopically stabilized ogival is actually a bit different). And if the projectile had thrust that exactly compensated the drag, it would indeed fly straight in all the frames of reference. However, don't forget that the thrust vector of a rocket missile is aligned with the launch platform, which is not aligned with the relative wind. And to adjust it to compensate for the drag, you have to turn that forward.

@Jan Hudec,  Why would you assume that the thrust is aligned with launch platform? Indeed, as soon as a rocket leaves it's launching tube, or rail, or whatever, it streamlines and aligns in its own relative wind. Unless it aerodynamically creates an asymmetric sideslip force, its thrust will always be aligned with the relative wind, opposite its free airspeed velocity vector. but a bullet, even if gyroscopically stabilized (in which case its spin axis will remain aligned with the launch platform, will still create drag opposite the relative wind, not the spin axis.

Indeed, due to angle of incidence of the launch tube or launch rail, a rocket fired from an aircraft is not always necessarily aligned with the relative wind at launch. But it does align as soon as it is in the free airstream, and so all thrust produced is in the direction opposite it's airspeed velocity vector. Throw an arrow out the window of a moving car and watch what happens. The arrow will immediately align itself with its relative wind. A rocket fired from a moving aircraft behaves in the same way.

I though the Bismarck had trouble with hitting the Swordfish precisely because they were so slow and the gunners thought they were moving faster?

While both the fighter and the bomber fly straight, the bomber for the bombing run and the fighter to keep its guns on the bomber, the bomber is far easier to spot for the fighter pilot, than it is for the gunners in the bomber to spot the fighter.
However, as the gunners of the bomber shoot back at the fighter, the fighter can't afford to take his time to get a good aim at the bomber. Thus, defensive fire from the bombers greatly reduced the fighters' time to take a shot at the bomber.
I have no figures on effectiveness, however, the Luftwaffe developed tactics specifically to avoid staying in the field-of-fire of the defensive guns. That should give some indication; if it is worth it to modify aircraft to avoid the defensive guns of bombers, then the defensive guns were considered a threat to the fighters.

Tactics adopted by the Luftwaffe to deal with USAAF bomber formations included head on attacks, which minimised exposure to defensive gunfire, and the use of rockets to engage the bomber formations from outside of the effective range of their .50 calibre machine guns.

The fighters also had more firepower. The bomber generally only had the .50 caliber (12.7mm) machine guns with solid bullets, while fighters mostly switched to 20mm or 30mm cannons with explosive ammunition, which did a lot more damage on hit.

@J.Southworth: This resulted in a bomber pilot making ace because he happened to have a nose gun while most didn't.

It seems improbable to me that a bomber pilot would have been able to destroy five fighters with one machine gun. I don't buy it. He may have claimed five, but it's been shown that claims by bomber crews for enemy aircraft destroyed were often exaggerated.

I read about one instance in which a Ju88 night fighter destroyed two P51 Mustangs during a day mission. The Ju88 was designed originally as a bomber, but developed into fighter versions as well.

The question of the effectiveness of bomber armament during the Second World War has to be assessed in the context of the bombing tactics used rather than in isolation. The tactics used by the USAAF units operating the B17 in Europe were based on the principle that bombers flying in formation could combine their firepower to effectively defend themselves against any attack. This proved incorrect, and heavy losses occurred, necessitating the provision of fighter escort. USAAF bomber losses on daylight missions were however lower than those suffered by RAF Bomber Command in night missions, in which the individual aircraft flew alone and if intercepted, had little chance against a heavily armed twin engined night fighter. Night bombing tactics were adopted by the RAF as a result of early combat experience in which their medium and heavy bomber aircraft proved excessively vulnerable in daylight missions, mainly because of their inadequate performance and defensive armament. At the beginning of the war, the official but nevertheless naive RAF doctrine was that bombers could effectively defend themselves against fighter attack if equipped with power operated gun turrets, even if these were only armed with rifle calibre machine guns.


text:
Why are many early gliders high-wing designs as opposed to newer designs?
Many early gliders were designed with a high-wing configuration. An example of this is the K 8, but many more of the same and previous generations follow this configuration.
Later generations (e.g., ASK 21) mostly follow a mid-wing design.
Is there a simple reason for this (including the change of material for fuselage and wings)?

It has to do with improvements in the understanding of aerodynamics and the various tradeoffs discovered over time. High wing has less visibility in turns, especially during thermaling. Low wing has better ground effect, but gives you less options when landing in a crosswind. High wing has better ground clearance, which helps in off-field landings.


text:
Are there any contemporary advantages in favor of wing warping?
The early Wright Flyer famously used wing warping for its roll control surfaces. This technology was soon abandoned in favor of ailerons and by the end of the biplane era, was not used anymore. I assume a good part of this was the greater amount of control that rigid control surfaces allowed.
There appear to be some experimental supersonic aircraft that are using wing warping (or wing morphing) technology. What is the perceived advantage of this new generation of wing warping technology, what are barriers to its adoption, and how practicable is it for it to trickle down to general aviation?

All wings warp. The extent of warping is one restriction which limits the fast side of the flight envelope.
Once aircraft designers realized that thick wings have less drag, they tried to build their wings as stiff as possible. Stiff wings allow to use airfoils with rear loading, will have higher flutter frequencies and keep their shape even when operating at high dynamic pressure.
Since the center of pressure moves chordwise on cambered airfoils depending on the angle of attack, any cambered wing will suffer from pitching moments around its axis of elasticity. This moment will twist the wing and change the incidence of the outer wing sections. A stiff wing will suffer less twisting and stay closer to its ideal aerodynamic shape. Add an aileron, and another twisting moment is added on top. Now the wing twists opposite to the aileron deflection! This can become so bad that most of the aileron's effectiveness is lost as the wing is warped in a way which counteracts the lift change of the aileron.
The X-53 research aircraft uses this effect with an array of leading and trailing edge devices to actively warp the wing in the desired direction. Using computer-controlled deflections of small but very effective control surfaces, it creates a desired deformation of the wing for roll control.
This can be compared to the effect a Flettner tab has on an aileron. The Flettner tab is a small control surface at the trailing edge of a larger control surface and is deflected by the pilot such that it moves the big control surface in opposite direction. It slightly reduces the effectiveness of the big control surface, but needs only a fraction of the stick force, thus helping to manually control big aircraft.
Since the active wing warping of the X-53 depends on the proper functioning of the FCS and an array of movable devices, each with its own failure mode, I would hesitate to see this as a clear advantage. It allows, however, to control the aircraft at high dynamic pressure with minimal drag penalties.

After searching for morphing I stumbled on to this question and just wanted to add a few things to what was already said since the question is a bit old, in case anyone like myself happens across it during a search...
Question: What is the perceived advantage of this new generation of wing warping technology, what are barriers to its adoption, and how practicable is it for it to trickle down to general aviation?
Answer: A warped wing or a morphing airfoil, shows significant improvement in lift to drag ratio and in decreasing the overall drag of the vehicle.  Depending on the specific technology many of the components are internal which has several benefits (decreased drag, increased life span) and with advancing technology and research the power consumption and additional weight of these components has been significantly reduced.  In the world of UAS's morphing wings have been show in some cases to increase stall AoA by 8 degrees or more (see Membrane Wing-Based Micro Air Vehicles) The complexity of analyzing morphing wings and airfoils however can present a challenge, as now there is significantly more fluid structure interaction, requiring solvers with more capabilities, or an increase in experimental data.  Large scale implementation is currently underway (see NASA Successfully Tests Shape-Changing Wing for Next Generation Aviation or Morphing Wing Passes Test, Business Jet Flew Concept Years in Making) and in a aerospace world that is concerned about improvements to develop faster and more efficient vehicles this technology holds some appeal.


text:
Who is the third pilot in this Qantas Airbus 380 long haul flight?
In the documentary, Secret Life of the Long Haul Flight (2017), we get a look at a long haul Qantas flight from London to Sydney.

One of the scenes shows the flight deck with three pilots (at least their epaulettes have chevrons on them). What is the third pilot in the middle doing? Just hanging out? If he's on rest, shouldn't he be in the rest quarters and not on deck?

I was just watching this yesterday and wondering free same thing. My assumption is it's just a third pilot. There are restrictions as to how long a pilot can be in command of an aircraft, so they must get some rest. In this case the third pilot is just with the others during certain moments of the flight. Alternatively it could be a pilot under training.

In the first photo, the 3rd pilot is holding an approach or departure chart. Standard ops for all pilots to be in the cockpit for these phases.

@TayE Pilots in training are seated in front and the examiner is seated in the third seat behind.

The two stripes on his shoulders indicate he is a Second Officer. They are fully qualified pilots but only act as cruise relief pilots on long flights. New Qantas pilots are  initially employed as a Second Officer.
It is standard practice to have all pilots in the cockpit for takeoff and landing. Their role in these periods is pretty much to just keep an eye on everything in case the other two miss something. Some time after the aircraft reaches cruising altitude one of the pilots will start their break and the second officer will take over.

On. 24hr long haul flight, wouldn't there be an entire replacement crew: a relief captain and relief copilot? (Four people total including the two in the cockpit). Or can you do with just three total (pilot, copilot, relief pilot).

It stops in Dubai, so I'm guessing they have a crew change there.

@RoboKaren Jeff is right, they stop at Dubai and will rest there for a day or two before flying to SYD/MEL on a different flight.

He's a Second Officer, and on-deck for takeoff, or landing. There is no such thing as a '24 hr long haul flight'. The maximum is about 15 hrs, in which case there is a captain, 2 first officers, and 1 (sometimes 2) second officers. They alternate their breaks. During cruise (only), if the captain is on break, there's an F/O and a 2nd officer in the seats. The 2nd officer doesn't generally touch anything - he/she is restricted in what they can do, and just monitors. It's a very boring job until they get to become F/Os!.


text:
Why is JetBlue's IATA code B6?
I understand why JetBlue's IATA code isn't JB since that belongs to a pre-existing helijet service in Canada, but why is it B6?
Is there some history behind the B6 moniker?

I've pinged JetBlue via Twitter, hopefully they have an answer.

@SargunDhillon Seems, they didn't! :>

@trejder Look at Sargun's comment on RedGrittyBrick's answer.

Software devs know - same reason we have k8s and i18n I suspect.

I don't know, but ...
If you look at the IATA application form you will see there is nowhere for the airline to indicate the code it would like to receive.
I imagine many new airlines might indicate a preference in an accompanying letter (e.g. they would like a code starting with a specific letter)
If you look at the allocated codes you can see that, initially, some attempt was made by IATA to make the two-character codes mnemonic
AA = American Airlines
BA = British Airways
etc

However if JB was already allocated, it is reasonable that the IATA
staff would look for an available code starting with either J or B.
Since the Blue of JetBlue is more distinguishing than the Jet part of the name, the IATA employee may have started with B and noticed that the following were already allocated
B1 Bravo Passenger Solution Pte., Ltd.
B2 Belavia
B3 Bhutan Airlines dba Tashi Air Pvt L
B4 ZanAir Limited
B5 East African Safari Air Express Ltd.

(note I have not checked dates these codes were allocated but the hypothesis seems plausible to me)
The last two are puzzling but perhaps they changed their names (perhaps as part of a takeover, merger or split). Airline codes do get reused. For example, B5 was used by FlightLine before it went out of business.
Then B6 would be the next available code.

I think you're right about it just being next in line. If you look at the years they were founded they go in sequence. I don't think the B has any significance since the airline was having major trouble coming up with a name and went through several, so they just got the next in sequence. If they had a name from the start they might have been able to get something specific but they apparently came up with jetblue at the last moment.

This seems correct: http://i.imgur.com/qcos5eW.png. The IATA naming process seems somewhat opaque.

VERY interesting. I always wondered why the Concord was SpeedBird, now it seems IATA staff are a bit arbitrary and capricious !!!

@jzumwalt Speedbird is the British Airways callsign, not limited to Concorde. It's Speedbird for historic reasons (the callsign of the BOAC, which was established before the IATA existed).


text:
Could older airplanes be retrofitted with engines with chevrons?
The 787 uses a variety of sound reducing mechanism including engines with characteristic chevrons:

(Source wikimedia commons)
Would it be possible for older airplanes to also have this type of quieter engine retrofitted? I'm assuming the engines would have to be recertified of course. But do the chevrons implicate a particular type of wing or airframe that requires them to be designed in from the beginning?

It's more economical to design and install "hush kits" for older engines. Engines are really expensive to replace (they represent about 25% of the sticker price of a $300 million airliner), then you have to consider replacing them on a fleet of dozens of these aircraft, with each upgrade taking that plane out of the sky for weeks in addition to the cost of the engines. Hush kits are usually much cheaper and easier to install (a few days maybe), though slightly less effective than an engine designed from scratch for lower noise.

The chevrons do not implicate a particular type of wing or airframe. However, the engine does need to be designed around having the chevrons, since they affect the performance as well as noise. And the engine design certainly can affect the design of the wing.
In reality, chevrons are more likely to be incorporated in an engine design from the start rather than retrofitted later. And a new engine design will probably be for a new airplane design. This could be a totally new plane like the 787. But it could also be a new derivative, like the 747-8 or the 737 MAX.
So while chevrons can be added to the engines on an existing aircraft model, it's much more likely that the aircraft model will be upgraded in other ways as well to accommodate the new engines. Adding chevrons is likely to be part of a design change large enough to justify changes to the aircraft as well.
However, these changes are not really specific to the engine having chevrons. Changes to the wing and airframe will be due to the inclusion of a new engine, not just the addition of chevrons.

Yes. This kind of engine design is independent of the wing and airframe and can be fitted on compatible aircrafts.
For example, General Electric GEnx-1B64, an engine with chevrons is fitted on Boeing 747-8, a variant of Boeing 747.
In general, the engines (especially civil airliner engines) are developed with compatibility in mind to reduce costs and increase commonality.

Not saying you're wrong, but the 747-8 has a new wing...

To quote 747 chief project engineer Bruce Dickinson, "fundamentally, the wing's structural design is not entirely different" from that of a 747-400, though some changes were made. I understand that significant changes were made (like raked wingtips, increased span etc.), but the airfoil section and the basic structural construction remains the same.

The Bruce Dickinson!? That's interesting, though. I thought they'd modified the airfoil.

You're also not going to see any of the older 747-400s coming back to the barn for GEnx engines anytime soon. New variants of older designs, sure; the 737MAX will have similar chevroned engine exhausts from a GE-Snecma joint venture. But engines on existing aircraft are usually only replaced if they're declared a total loss due to accident (engine fire, bird ingestion, blade-out) in which case only the destoryed engine is replaced, in turn meaning it has to be a like-for-like swap.

@KeithS that's not entirely true. There are several major re-engine projects I can think of - the KC-135R and DC-8-7x both got CFM-56 engines late in life as refits. Turboprop upgrades have happened as well, including planes like the DC-3.

On military planes, yes, re-engines are more common, but in jet airliners, a fleet-wide re-engine of an older model isn't going to pay for itself before the entire airframe is retired.

As far as using the GEnx-1B on the 747-8, not entirely correct. They can use the 2B-67 which has a 6" smaller fan diameter, but most importantly it has bleed air which is needed on the 747 lineup.

@KeithS: The DC-8-70 is a civilian jetliner (or, at least, it was - nowadays, the few remaining are used as freighters).

@egid: The DC-3 isn't really a fair comparison, given that it's an unpressurised aircraft with an essentially infinite service life.

Yes.  The parts shown with the noise reducing 'chevrons' are part of an assembly called the engine cowling, which is usually manufactured by a separate manufacturer than the engine.  It is reasonable to assume new engine cowlings could be designed as part of an STC upgrade.  NORDAM and UTC Aerospace Systems (Goodrich) are examples of engine cowling manufacturers


text:
Can ekranoplans (GEV) be more efficient than traditional airliners?
Ekranoplans or Ground Effect Vehicles (GEV) fly low enough to the ground that their wings stay in the ground effect. This increases lift and efficiency.
But GEV fly in the lower atmosphere (literally ground level) which greatly increases drag compared to flying in the stratosphere.
Is it possible that a GEV designed using contemporary composite materials and high bypass engines could be more efficient per cargo weight-mile (or passenger mile) than a tradition airliner? Or are GEV inherently less efficient due to the altitude they fly at?
This question is specific to fuel efficiency over distance. Feasibility, safety, navigation, time, and cost of airframe are not part of the equation.

This nasa study suggests GEV could be more efficient but it’s light on details: https://nari.arc.nasa.gov/sites/default/files/attachments/IFAR_AeroAcademy_2014.pdf

.. but it’s the reason I ask the question because at least my received wisdom is that GEV were inherently inefficient except for very short hops such as crossing the Caspian Sea.

I always thought that the extremely low aspect ratio negated most of the ground effect benefit from an efficiency perspective.

@JohnK, no. Much of the ground effect per se manifests itself in reduction of induced drag - which is exactly the reason to have long wingspan on 'normal' airplanes. GEV simply don't need long wingspan: they achieve low induced drag by other means. Furthermore, ground effects generally scale with wing chord, so GEV benefit from wider chord.

You need to specify the trip length, as it takes time to climb to altitude. A GEV would be more efficient for trips where there isn't time to reach altitude.

@RobinBennett as long as its apples to apples, I’m fine with any distance. So short hops across the Caspian as well as across the Atlantic. I’d just like to see hard numbers.

The magic number for airline operations is transport performance, adding speed as a factor. This higher speed is best achieved in thin air. With the same amount of friction drag as a ground effect vehicle, the airliner travels about three times faster, requiring a much smaller fleet size for the same transport performance. Just another one again.

As some of the comments above (which are really on the money) suggest it's a complicated question to answer because it is a case of "horses for courses" and that is even if we focus purely on straight-forward fuel efficiency. But given the programs that have been attempted (or are still ongoing) and also various studies it seems that ekranoplans seem to work best in two very distinct roles. Either as small, short range puddle-jumpers or as very large trans-oceanic monsters. For the later see also this which may well come to pass.
I guess at some level such utterly different roles is a problem for ekranoplan development. But, yes, in principle, there would certainly appear to be certain, very different niches, for which the ekranoplan is extremely competitive in terms of fuel consumption. Having said that the second niche is arguably also competing against conventional surface sea-freight which of course complicates matters but offers interesting possiblities. Aircraft like the Boeing Pelican (which didn't get past concept stage) was designed not for standard air-cargo pallets but for ISO containers. Now that capability is a real game changer as long as the cost per kg/km fuel economy is as good or better than a conventional air-freighter which I suspect is very doable. At least in principle.

Fuel efficiency of a given type can vary considerably over different distances. The conditions necessary for optimal efficiency also vary widely between types. The jet is best at high speeds and high altitudes, so acceleration/climb and deceleration/descent are important considerations on short journeys. The piston engine is more efficient at low altitudes, but its propellers make for slower airspeeds and this makes long journeys unattractive.
A jet-powered GEV is definitely less efficient than a high-altitude jet, but a piston-engined GEV is in its element and can also make use of ground effect to increase lift and  improve aerodynamic efficiency. Theoretical arguments one way or the other are one thing and opinions differ, but whether or not it can prove more efficient for such routes than a conventional airplane must wait for practical demonstration.
But the ekranoplan suffers a curious limitation in that its maximum speed over water turns out to depend on its wing chord, or fore-aft measurement. Wave interference effects mean that the longer the chord, the faster it can go. Above its maximum interference-free speed, its efficiency is among the performance criteria which fall sharply. Thus, the very large but relatively short-range ekranoplan is potentially the most efficient. This was the niche at which the Caspian Sea monster was aimed.
Of course, finding an obstruction-free route for any distance over land is next to impossible.


text:
What is the immediate cause of stall?
The direct cause of stall is unclear to me.

I heard about exceeding maximum angle of attack (around 40°) ;
I heard about reaching the stall speed in the current configuration (flaps, etc).
I heard about load factor which can lead to a stall.

Are these three parameters both direct causes of stall? By "direct", I mean for example that pitch is not a direct cause to stall, since pitch let AoA vary which causes stall.
It's unclear because there is a relation between angle of attack and speed. When we increase the angle of attack, we lower speed. But at the end, what will be the cause of stall?
If both causes are direct causes, are they independent?
- It should be possible to fly very fast (faster than stall speed) and have an angle of attack of 40°.
- It should also possible to have a very low speed (lower than stall speed) and have an angle of attack which produces lift.
What parameter will win and how are they related together?
When an aircraft is on the ground, taxiing at 10kt, the speed alone makes the plane stall, regardless of the angle of attack, am I right?
I would like to know necessary conditions to stall, and sufficient conditions to stall (all conditions being direct causes).

I would like to know necessary conditions to stall, and sufficient conditions to stall (all conditions being direct causes).

those 40° are only valid for a low aspect ratio, swept wing with large strakes. The stall angle of attack with straight wing aircraft is closer to 15°.

The immediate cause of a stall is the detachement of the airflow from the wing:

Image from NASA
This happens when your Angle of Attack is too high.

How can the AoA become too high?
--For level flight:

Given a certain velocity, you will have a certain AoA that will provide the lift needed for level flight. The lower the velocity, the higher this AoA.
Given a certain wing (configuration) you will have a certain maximum allowed AoA (in the picture is the AoA for wich you have the highest value of the plot):

Image from Wikipedia
Given these two tidbits of information, you can see what is the answer to your question:

If the velocity is too low, to fly straight you need an AoA that exceeds the capabilities of your wings (or a vectorial thrust that exceeds the weight of your veichle, but I would say that it is not quite common)
If the AoA is too high, well, is quite evident that you have stalled.

--For curved flight:
During a turn you need to use a component of your lift vector to actually turn

Image from here
As you can see the lift you need to turn is larger that the lift you need to fly level (given the same speed). And to have a higher lift you need a higher AoA.
And once again back to the AoA we are: if you want to turn too much (high load factor) you need a lot of extra lift, meaning that you have to increase your AoA.
If you exceed your wing limits, you stall

Dynamic stall
This is not exactly my cup of tea, but reading from the already linked wikipedia article:

Dynamic stall is a non-linear unsteady aerodynamic effect that occurs when airfoils rapidly change the angle of attack. The rapid change can cause a strong vortex to be shed from the leading edge of the aerofoil, and travel backwards above the wing. The vortex, containing high-velocity airflows, briefly increases the lift produced by the wing. As soon as it passes behind the trailing edge, however, the lift reduces dramatically, and the wing is in normal stall.

I would say that once again we fall in the "too big AoA" case, even if for different causes.

But flight manuals often talk of "stall speeds". Actually, from what I understand of your answer, speed is not a direct cause of stall. We say low speed stalls the aircraft because we mean "if we want to fly straight and maintain that low speed, we will increase the AoA, hence the stall".
Is it theoretically possible that we lower the speed but we don't increase the AoA? But the plane will descend, and the airspeed will increase again...

@Fox yes, you can descend like you say, but I would not call that (controlled) flight anymore.

Is angle of attack the only cause of stall? What about dynamic stalls and stalls caused by high load factors?

@Fox: When you lower speed and don't increase AoA, you will have less lift and because gravity stays the same, you will be accelerating downward. So you can't maintain the lower AoA for long. There is still good use for this; if you loose engine power, you need to retract flaps to reduce drag. And the fastest way is to push down to zero G/zero AoA (until you start floating in your seat), immediately retract the flaps (as the wing won't stall when it's not generating lift) and pull out when you accelerated to $V_Y$.

There is not much to add to the two good answers, but I'll try to refine them. The main reason for a stall is flow separation and, accordingly, lift loss beyond the angle of attack of maximum lift. Unfortunately, this is not a fixed number, but it depends on a range of parameters. The three most important ones are Reynolds number, rate of pitch increase and Mach number.
The Reynolds number characterizes the ratio of inertial and viscous forces in a fluid. In other words, friction has more influence on airflow at lower speeds and smaller dimensions. Friction is the main reason for flow separation (please read the linked article to get an explanation), and the slower the wing moves through air, the smaller the stall angle of attack is. In a taildragger taxiing at 10 knots, the wing is certainly stalled.
In flight you need to factor in how much lift you demand from the wing. This is determined by the product of aircraft mass and load factor. If you fly a parabola with zero g, you need no lift, and the aircraft's wing will not stall at any speed. Note, however, that the elevator might need to create substantial lift in order to keep the wing close to it's zero-lift angle of attack, so the elevator might stall at low speed. You will notice when this happens, because your parabola will come to a sudden end.
On the other end, flying a steep turn might cause a stall even at high speed, if you pull too many gs for the given speed. This is just the same as a stall in level flight at low speed. Due to Reynolds number effects, the stall angle of attack might be a few degrees higher, but details depend on the particular aircraft and it's airfoils. Generally, your stall speed in a turn goes up with the inverse of the square root of the cosine of the bank angle. In a 60° turn, your stall speed is 1.41 of your level stall speed, and at 75° it will be almost double your level stall speed.
The pitch rate can have a dramatic, but short-lived influence. See this post for details. In tests, the maximum lift could be increased by 50% simply by quickly pitching up. If the stall AoA is approached rapidly, the boundary layer over most of the wing has still the characteristics which go with the low AoA that prevailed when that parcel of air flowed around the wing's nose. Once that boundary layer has been washed away, the airplane is deeply in stall territory and needs to pitch down a lot to recover. Pitch fast enough, and normally benign aircraft can show dangerous stall characteristics. This is fun to try, but make sure you have enough altitude below you to recover.
And now for the influence of Mach number. Again, this post has more details (scroll down to the lower five paragraphs). Once part of the flow over the wing becomes supersonic, maximum lift suffers and the stall angle of attack is lowered dramatically. This is a high-speed stall, and it can become hard to recover. Stalling means a loss of lift, so the aircraft will pitch down, picking up more speed. This will drive it deeper into the high Mach region with severe shocks on the wing, so by accelerating it will make the stalling condition worse.
Especially high altitude aircraft can get into a condition where they fly right between the low-speed and the high-speed stall. Decelerating means increasing the angle of attack above it's maximum, and accelerating means the shocks on the wings get worse, reducing lift and forcing the aircraft into a prolonged shallow dive, until air density is sufficient for recovery. U-2 pilots called this the coffin corner of the flight envelope.
Wing sweep increases the maximum angle of attack, and with leading edge sweep angles of 60° and more, flow separation at the leading edge creates a vortex which increases lift with higher angles of attack, so that a normal stall does not occur. Of course, pitch high enough and the vortex will become unstable, but increased drag, separated flow around the vertical tail and vortices on the forward fuselage will limit how high you can pitch. The F-4 Phantom II has a maximum angle of attack at just 23°, where the wings will happily produce a positive lift gradient with angle of attack. But the vertical tail rapidly becomes ineffective beyond those 23°, and the aircraft will violently yaw out of control if that angle is exceeded (nose slice). Effectively, aircraft like the F-4 never stall, they just go out of control if you pitch up too much.

Stall means that the wing has exceeded its critical angle of attack. Nothing more, nothing less.
Is it possible to fly very fast beyond critical angle? Sure. Look up Pugaychev's Cobra. Or look at the Saturn V rocket. Both aircraft are stalled, yet they continue to fly. My previous example was flawed, but the sentiment stands.  Put enough thrust behind a brick outhouse and you can get it to fly. (Thanks newmanth!)
What about flying slowly beyond stall? That won't work; beyond critical angle you develop less lift the more you increase AoA. For any given airspeed / wing / air density configuration, there is some maximum lift value which cannot be exceeded. So the more slowly you fly, the more down you go.
Your example of an aircraft taxiing at 10kt is an excellent one. Just because the airplane is moving slowly does not mean that the wing is stalled. There is 10kt of relative wind passing over the wing, and the angle of attack is close to 0. The wing isn't stalled, it's just not generating enough lift to fly.
To reiterate, stall means that the wing has exceeded its critical angle of attack. That's it.

Your answer let me realize where is my confusion: I was mixing low lift (lift is not enough) VS stall, which is only airflow separation from the wing.

I'm not sure that Pugachev's Cobra or the Saturn V are appropriate examples, as neither are using an airfoil to generate lift (since both rely on the direct effect of thrust instead). At least in the case of rockets, any use of airfoils is for stability control only, and these are actually maintained at a near ZERO angle of attack.

@newmanth - Thanks!  I've edited my answer to correspond to your comments.

"Put enough thrust behind a brick outhouse and you can get it to fly". To fly (lift required) or to be propelled (no lift required, but more fuel needed for the same trajectory)?


text:
Does an IFR flight at night in VMC count as “night VFR conditions” (FAR 61.129)?
FAR 61.129 several times uses the phrase “night VFR conditions.” Does this mean strictly flights conducted under VFR at night, or does it include night flights conducted under IFR but in VMC?

"Night VFR conditions" describes the flight conditions, not the flight rules. The PCG says:

VFR CONDITIONS− Weather conditions equal to or better than the minimum for flight under visual flight rules.

If the requirement was to conduct the flight under VFR, the regulation would say "under VFR". For example, 91.185(b) shows that the FAA sometimes makes that distinction (emphasis mine):

If the failure occurs in VFR conditions, or if VFR conditions are
  encountered after the failure, each pilot shall continue the flight
  under VFR and land as soon as practicable.

So you can do the flights under IFR in VMC if you like.

Since it specifically says "VFR," you can't be on an IFR clearance, or following IFR rules in uncontrolled airspace.  You are never both VFR and IFR at the same time, you can only be one.  You can be VMC while IFR, but that doesn't meet the requirement as written in this rule.

When a pilot is flying "VFR on top", they must comply with VFR rules (cloud clearance; own traffic management) and IFR rules at the same time.

Can you cite a source for this? The phrase "VFR conditions" is actually quite ambiguous. VFR is a set of rules, not a condition. VMC is a condition. They should have said either "in VMC" or "under VFR."

The Aviation Weather Center (at https://www.aviationweather.gov) describes weather conditions as "VFR", "marginal VFR", "IFR", or "low IFR", even though those terms don't accurately describe the rules flights are actually conducted under.

I have LONG hated the phrases VFR/IFR "Conditions". I think they are vague and lead to misunderstandings. They are used in multiple locations throughout the FARs, and IFR Conditions is even defined in Part 1. This question/answer is a specific example of it being a confusing term. @Ralgha says, "it specifically says VFR"; however, that's not correct. It says "VFR Conditions", which is a set of wx conditions that differ depending upon class of airspace. So as long as you have "good weather", and you're Inst. rated, you can file IFR and meet the strict verbiage of 61.129 for your night flights.


text:
Is this a C-17 (and why is it here)?
This aircraft flew overhead at less than 1000 ft a little while ago. We think it's a C-17. We can't figure out what it was doing...

(Click here for full image)
We're in Culver City, between LAX and Santa Monica. It was traveling south to north, about 4:50 this afternoon.
Scared the neighborhood...

Why were you scared?

That is way more than 1000 feet.

the whole neigborhood, huh?

They aren't quiet aircraft.

Yes, that is indeed a C-17. As to why it's in the area, only the Air Force would know.

https://www.boeing.com/defense/c-17-globemaster-iii/#/overview

The images don’t do justice to its altitude. Nor the fact that it was banking right then left .. Exciting..

@DavidDelMonte It’s not a big deal, dude. Edwards AFB isn’t far by flight and there are plenty of military installations in the area that it could be practicing with. It could be anything: Touch and go’s, dropping paratroopers, doing search and rescue training.... Or, you know, chemtrails...

For what it's worth, I've seen them first hand, so that's why I was confident enough to say that it is indeed a C-17.


text:
Covid-19 - Why are there still a lot of international flights?
I hope this question is relevent to Aviation. If not, please let me know. Below is a snapshot from Flightradar24. While there are far less flights than usual, there are still a lot of flights en-route.  I thought there were controls put in place last week in several countries. Do these flights have special permission?

This may be a better question for the Travel Exchange. There are controls and restriction put in place. That is different than a total prohibition on flights like the type we saw after 9-11.

The restrictions are on who can still enter a country (pass immigration), not which aircraft can fly where, so this would be a better question for travel.stackexchange.

Plus, you can almost treat Europe as one country with the EU. As far as the US, that is not a lot of international flights. It might not be a lot of total flight all together. Considering DFW alone gets 1000 flight cycles every 4 hours of a normal day.

Thanks Dean. I'll take your comments as an answer, and close the question. David

Barring foreigners from entering a country is not the same as saying planes are not allowed to fly. A lot of people abroad need to travel home, and a lot of cargo is (and will continue) being moved around

That said, the number of flights is already dropping drastically, and will continue to do so. https://www.flightradar24.com/data/statistics

People working and studying in countries with medical infrastructures less prepared for the epidemic are trying to go home.

The simple answer is that no one has shut down any airspaces yet and that is what stops planes from moving.
While many governments have put travel bans in place for people airplanes often carry other things like mail and cargo and it may still be viable and profitable to fly them. There is also the compounded fact that many people are trying to get home to quarantine them selves and in the short term there is still high demand for flights to move people back to their home countries.
Airlines are starting to cut flights due to reduced demand and over the next few months you will see a drop in flight traffic but if the airspace is open planes will keep moving.

Great comments and answers. Many thanks to all. Roger and Out. (I'm old enough to say  that)


text:
Does ICAO require (or suggest) set of technical statuses of radar and its subsystems?
Does ICAO require (or suggest) set of technical statuses of radar and its subsystems? In other words is it required that radar status can be set to Normal, Warning and Error, or Idle, Alarm, Unknown also possible? There are 002 and 034 categories of ASTERIX which describe messages about radar status, but I haven't found out answer on my question.

Welcome to Aviation.SE Glutton. Where did you find the statuses you mentioned? I can't find them in the ASTERIX specs.

Welcome to Aviation.SE, Thanks! Where did you find the statuses you mentioned? I've meant Data Items: I002/050 (Station Configuration Status), I002/080 (Warning/Error Conditions) and I034/050 (System Configuration and Status). But compliance with the requirements of the ASTERIX isn't my purpose, I just try use ASTERIX as a hint for my more general question.

These statuses are not defined by nor required explicitly by ICAO.
The details of how a radar (or other surveillance) system communicates with downstream ATC systems are defined in a technical specification documents, typically produced by EUROCAE and/or RTCA.
The ASTERIX specifications are maintained by EUROCONTROL and only give the format in which surveillance ground systems communicate amongst each other. The specifications were originally intended to facilitate surveillance data exchange between European countries, but they have now been adopted by many countries outside Europe.
Category 002 (old) and 034 define the status messages monoradar systems use to update downstream systems (radar trackers, displays) on the radar's status.

These statuses are not defined by nor required explicitly by ICAO., Thanks this is answer on my question. technical specification documents, typically produced by EUROCAE and/or RTCA, Thanks for hint.


text:
How long is spin training good for in the USA?
Written tests are only good for 24 calendar months before they must be applied to a certificate (or lost).  Spin training is a requirement of the flight instructor certificate.  Does the required spin training have the same limitation of life as a written test?  Or, is it like an endorsement where it's good "forever"?

CFI candidates receive a spin training endorsement with no expiration date. According to AC 61-65E (the endorsement appendix), it should look like this:

46. Spin training: section 61.183(i)(1).
I certify that (First name, MI, Last name) has received the required training of section 61.183(i). I have determined that he/she is competent in instructional skills for training stall awareness, spin entry, spins, and spin recovery procedures.

/s/ [date] J. J. Jones 987654321CFI Exp. 12-31-05

The basis for this is that 61.183 doesn't specify a time period in which the endorsement must be received:

§61.183   Eligibility requirements.
To be eligible for a flight instructor certificate or rating a person must:

[...]
(i) Accomplish the following for a flight instructor certificate with an airplane or a glider rating:

(1) Receive a logbook endorsement from an authorized instructor indicating that the applicant is competent and possesses instructional proficiency in stall awareness, spin entry, spins, and spin recovery procedures after providing the applicant with flight training in those training areas in an airplane or glider, as appropriate, that is certificated for spins; and


text:
Who owns B- registration? China or Taiwan?
Aircraft registration is unique to each aircraft. Each one begins with country code follow by numbers or characters. However wikipedia says that Taiwan and China both use prefix B. So who actually own the right to use prefix B? If two of them do not co-ordinate the use of prefix them there can be two planes of same registration in the sky. So how things work out here.
https://en.wikipedia.org/wiki/Aircraft_registration
https://en.wikipedia.org/wiki/List_of_aircraft_registration_prefixes

I assume you're aware of the complicated history of Taiwan being considered (by some) to be a province of China?

@Ben yes i’m aware of history from WW2 day to the day taiwan gott kicked out of UN security council till now. It’s complicated and out of topic to talk about jere.

From your second link:

Aircraft registered in the People’s Republic of China traditionally
have had "B-" followed by four digits.
B-0000 to B-9999

Since 2018, the following registration schemes are also in use:
B-000A to B-999Z (General)
B-00A0 to B-99Z9 (General)
B-00AA to B-99ZZ (General)

And for Taiwan:

B-00000 to B-99999 after 1990s

so there won't be any confusion, one has 4 digits/characters after the dash, the other 5 digits.

As a software developer, I wonder if there was ever a confusion/mixup between a B-ABCD and a B-0ABCD registered aircraft.

@DeepSpace, everybody knows the registrations are alphanumeric, that is text, and doing partial conversion to number like you suggest would be extremely contorted, so I would say no.


text:
How to request the programming of ICAO 24bit aircraft code into a PLB
I have my ICAO 24 bit Hex code programmed in the aircraft ELT (Cirrus SR22) and of course, it is also coded in my aircraft Transponder. I will purchase a COSPAS/SARSAT PLB (Personal Locator Beacon) for extra safety as I will be flying over the water.
I can opt for keeping the standard serial number for the PLB (which basically means it will be treated as a personal device, not processed as an aircraft ELT) or code the 24 bit code in the PLB (As of ICAO, this means SAR will consider the distress call as an aircraft call, which is what I want). PLB distributors are used to code marine vessels MMSI identificator in the PLB but know nothing about ICAO Hex code.
In which format should I send them to code the ICAO code in the PLB? Hex? Binary? Octal?
i.e.: Hex is 6 char hexadecimal alphanumeric code.

It only takes a few seconds to convert from one format to another, so just send them the hex code and label it specifically as a hex code. (This will usually be obvious anyway, as an octal code would only be 0 through 7 while a hexadecimal code will be 0 through F.)  If they actually code the device in something besides hex, they will know how to convert from hex to that format.

Thanks. I had prepared the code in three formats, Hex, Octal and binary, but looks like these guys are not that current with conversions as they are used to just code, at most, Marine MMSIs into the PLBs. Anyway, after some thought, I have decided to treat PLB as personal, even if used in flight, and not set the aircraft ICAO code in it. If the plane ditches I prefer SAR to be looking for a floating person and not an aircraft... but that's another subject...

@icordoba I think in the long term there’s more benefits to a personal code.  For me, I’d have more use cases for it than just aviation, and more aviation use cases than just my specific airplane.  If I bought a PLB, I would potentially carry it with me on airline trips, flights in planes with others, boating trips, and potentially other uses like hiking or skiing.  Quite often there are many uses for something that don’t justify the purchase, and yet if I am going to have one anyway, I’d like it to cover those uses.


text:
How do I plot Cd vs Re graphs for an airfoil?
I am looking for a platform where I can plot the value of the drag coefficient of an airfoil for a range of Reynolds numbers (Cd vs Re plots). I wish to analyze the trend of Cd with changing Re and compare different airfoils based on the same. I tried:

Airfoil tools but it only gives me Cd vs Alpha.
Xfoil but as far as I remember it doesn't plot data with respect to Re either

Is there any other platform or software where I could get this?
Some help would be really appreciated!

You had the right resource in airfoil tools but didn't use the comparison feature. Here are two airfoils as an example. To compare them go to the detail page for each and then click 'add to comparison' at the top right. You can select existing airfoils or add your own.

You will get a combined page with selection of Re for the airfoils selected. Pick the ones you want so as to limit complexity.

Then hit update plots and you get:

You have to use alpha as cd varies substantially with this, but the comparison is straightforward. In this example the blue lines are the two airfoils at re=50,000 and the brown/green are re=1,000,000.

XFOIL gives you several options, however, the X and Y axes will always be angle of attack or a coefficient. You can, for example, choose a Reynolds number (Re) which is variable with lift coefficient, so the resulting lift-drag plot is for a constant product of lift coefficient times velocity squared. The resulting drag coefficient at each lift coefficient then is valid for the same airplane at different speeds at the same altitude. There is no better way to compare two airfoils for the same airplane design.
If you desire to plot drag over Re, the lines would be valid only for one lift coefficient. It is more useful to plot several lines for the coefficients of lift and drag and to use Re as the parameter for the family of curves. Since friction drag changes roughly with the logarithm of Re, plotting is best done for magnitudes of Re (like 1, 2, 5, 10, 20 x 10⁶).


text:
Is there a classification/group for UAVs with certain specs?
Is there a classification (like a name for UAVs that have max cruise velocity of 350km/hr) for UAVs please? So based on specs like range, velocity and altitude for example?
Thanks, any resources with classification would be appreciated too.

Regulatory?  What nation/s?

So more how to classify specs, so let’s say group or something to classify all UAVs that have a range of 500km and less for example

Be more specific about what you mean by "classification system", anyone could make up any arbitrary classification system they want based on those same performance numbers. That would not be a useful question and answer as it is unbounded. I am guessing you want to know about one of the following examples: legal certification, traffic control, engineering standards, industry sales categories. Please edit the question to narrow the focus of the question.

With regard to certain specs? Not really.
Based on their roles they are loosely called MALE, HALE, UCAV, SUAS or whatever the manufacturer wishes to advertise them as. Since UAVs are fairly recent and a small part of aviation currently, legal classifications haven't been established yet.
If it helps have a look at 2021 notification by Government of India here which attempts to loosely classify them on weight


text:
What is the Eurofighter's altitude record?
The Eurofighter Typhoon is a pretty nimble fighter jet, and many European countries use it in their military. Its service ceiling is 65,000 ft, but what is its current altitude record from a zoom climb (i.e. apogee record)? How high can a Eurofighter get in a zoom climb, assuming ideal weather and maximum lightness by having little fuel left and no missiles on it? Would you be able to reach an apogee above 95,300 ft (29 km) in a zoom climb?
Also, what is the ideal altitude for a Eurofighter to start a zoom climb from, to get as high as possible? Answers/comments based on personal experiences are welcome as well.


text:
Is the U2's 74,000 ft the highest a turbofan plane ever has gone?
What is the altitude record for a turbofan? The (usually) turbofan-powered U2 has gone to 74,000 ft (22.6 km) MSL, but did any turbofan plane surpass that, in a zoom climb for instance and with afterburners' help?

Related: https://aviation.stackexchange.com/questions/45781/can-high-bypass-turbofans-cruise-at-high-altitude

The SR-71 goes higher, but its engines are in ramjet mode.

@JohnK Yes and it's a turbojet and ramjet, my question is about turbofans (some bypass ratio above 0).

To have a bypass ratio above 0 you need to have more air than you need in the combustion chamber. That's why you can afford to let air "bypass". This is far from true at that altitude.

@user3528438 Most U2s have turbofans. A high altitude can be achieved on an afterburner as well (if the plane has any, the U2 doesn't), and eventually with engines shut off, being carried by momentum to apogee.

mig 31 foxhounds regularly do 80,000 ft+ with a low bypass turbofan

@Mridul: Feel free to make that an answer.

No.
In 1975, the F-15A Streak Eagle, propelled by 2 afterburning turbofans, climbed to 98,425 feet in 3½ minutes, coasting nearly up to an altitude of 103,000 feet.

I am not entirely certain where you have your information from. The U2's service ceiling was 70.000 feet and I can't find any reports about it going higher.
The F-104 mentioned in @bjelleklangs answer uses a turbojet engine so that disqualifies it from your question. The F-15A Streak Eagle mentioned in @Bob Mathews answer would be a valid contender but I want to mention the McDonnell Douglas F-4 Phantom, which had turbofan variants for the Royal Navy and the Royal Air Force and reached an official maximum height off 98.557 feet a tad bit higher than the F-15. Most likely not with the correct variant though.
Both these planes definitely used afterburners. The highest altitude ever reached by a plane simply powered by a propeller, which is a category I would put turboprops under, without an afterburner was NASA's Helios HP01 at 96.863 feet.

The F-4 is a contender, but I was correct in using “streak” vice “strike”. The F-15 Strike Eagle is a completely different variant of the F-15 — the “E” model. The F-15 Streak Eagle was a specially-configured F-15A, and is the only jet with the name “Streak Eagle”. There were 513 “Strike Eagle” jets built.

@BobMathews You indeed were, my bad

No big deal. I had actually forgotten about the difference myself until I started looking at old articles about the Streak Eagle and the team of pilots who flew the record-breaking flights.

What's posted here are basically comments on other posts, a discussion in other words, and not an answer to the stated question.

I disagree with other answers and then supply additional information that answers the question. How is that not an answer?


text:
What is the AeroVironment Helios' Kármán line?
If it's true that the NASA/AeroVironment Helios maintained level flight 96,000 ft above sea level at a maximum airspeed of 23.5 kts (43.5 km/h) I assume it must have a very high Kármán altitude, the altitude at which it would have to fly at orbital velocity (15,100 kts) in order not to stall. By contrast the SR-71 Blackbird e.g. couldn't even fly at 23.5 kts near sea level. Is there a way to figure out the Helios' Kármán line, or any plane's for that matter? Known is only that of the X-2 which is 57 mi (91.7 km) according to Von Kármán's calculations himself. The X-2 flew  at 2,000 mph (3,200 km/h or 1,740 kts).

Related

This really looks like a better fit for Space.SE, to my mind.

This shouldn't move to Space.SE because it's about aerodynamics: how thin must air be for Helios to need 15,100 knots to get enough lift to support itself;  then, how high that is.  This may be for a fictional Helios, though, because its solar panels might not give enough power, and its propellers might fall apart at that many RPM.

I’ve always felt that the principle of the Kármán line is somewhat flawed; at orbital velocity an object can maintain altitude without any aerodynamic life at all, so a little below this speed it would need to generate only a small amount of lift.  My understanding of the Kármán line principle is that this is the altitude at which an aircraft couldn’t generate enough lift to support its stationary weight, which of course it wouldn’t need to when moving at best-orbital speed.  In the case of Helios the situation is rather clearer cut, since the aircraft travels at a speed far below orbital.

Mach Limited Airframe.

There are no different Karman lines between the two aircraft. The Karman line starts at ~62 miles (327,362 ft) above MSL, an altitude neither aircraft can attain.  In addition the Aerovironment Helios had a wing loading of 0.69 lbf/ft^2 whereas the Blackbird’s wing loading is 84 lbf/ft^2.  As such, I’m not surprised at all that a Blackbird cannot maintain level flight at 23 KIAS but a Helios can.

The 62 miles are actually a rounding of the FAI's 100 km (62.14 mi / 328,100 ft) line which they established as "the" Kármán line. In fact, the line is different for each aircraft and varies by other factors as well. Von Kármán himself determined 91.7 km (57 mi) for the X-2 as stated. These are theoretical values because it's clear no plane can fly horizontally at those altitudes, only the Space Shuttle and the Buran could during reentry.

@Betternottell The X-15 got to almost 315,000 ft.

@Betternottell No, the line is not different for each aircraft. Check the wiki: "The Kármán line (or von Karman line) is an attempt to define a boundary between Earth's atmosphere and outer space". Further along in the article one can read that he used the X-2 to illustrate an example of division of lift & Kepler force. He did not state that the calculation was specific to the X-2 only, it is indeed a singular line for all aircraft.

@Koyovis No, it's dependent on the stall speed at altitude of each aircraft.

@Betternottell Eh - no. As can be clearly distilled from any article on this topic.

@Koyovis When they talk about the Kármán line they usually mean exactly 100 km (62.14 mi), the number established by the FAI.

You guys are all wrong here. You’re confusing a predefined Karman line with Von Karman’s  attempt to calculate a maximum theoretical altitude of an aircraft could fly at.  There are two totally different things, as the wiki article points out.

@Betternottell, your last two comments appear to be diametrically opposed…

@MichaelHall In the 2nd comment I wrote about what the FAI refers to as "the" Kármán line. There is not a single Kármán line, for it depends on the stall speed of the plane in question. Von Kármán determined a 57 mi (92 km) value for the X-2 while the 100 km value has been made up by the FAI. They wanted a nice round number so they rounded, but in the wrong direction as they should have rounded to 90, not to 100 km. Or should have used nautical miles (50 nmi) which would be closest.

Do you have a reference for your claim that it depends on stall speed? Because I’m new to the term, but that doesn’t mesh with common sense in context with the definition I have read. And there are people here whose answers I respect who disagree with you…

Pic from the wiki
The Helios is a slow moving aeroplane, while considerations regarding the Karman line are about very fast moving aeroplanes. From this link:

...the Karman line is determined by calculating at what elevation the Earth’s atmosphere becomes too weak to support flight. At the Karman line, the atmosphere is too thin to support flight, and the plane must go fast to stay aloft. The Karman line is situated above the Homopause, and above this point, the atmospheric gasses are not well-mixed.

So the Karman line is the line of orbital velocity where very little to no lift force is contributed by aerodynamics. Orbital velocities are very high, and high speed is not something that the Helios is capable of, it can remain aloft by aerodynamic lift only. Discussions about Helios and Kármán line are incompatible.

The orbital velocity at any altitude of any aeroplane or circulating object is purely a function of its maximum speed. Considering a simplistic situation (homogenous spherical earth etc.) in the above pic, the centrifugal force F$_c$ must be  equal to gravitational force F$_g$:
$$F_c = \frac{m \cdot V^2}{r} = \frac{m \cdot V^2}{R + h}$$
$$F_g = m \cdot g_h = m \cdot g_0 \cdot \left( \frac{R}{R+h} \right)^2$$
With $R$ = earth radius, $h$ = altitude, $g_h$ = gravity at altitude
Equaling the two equations, and computing for several altitudes, results in:

There is no aeroplane that can fly that fast, not even the SR-71 or the X-2 which only reached about 1,000 m/s @ 24,000 m. They require aerodynamic lift!
So the Kármán line is not aeroplane dependent, it is the boundary where aerodynamic flight is not possible anymore by any aeroplane. From this site:

The Kármán line is based on physical reality in the sense that it roughly marks the altitude where traditional aircraft can no longer effectively fly. Anything traveling above the Kármán line needs a propulsion system that doesn’t rely on lift generated by Earth’s atmosphere — the air is simply too thin that high up. In other words, the Kármán line is where the physical laws governing a craft's ability to fly shift.

Did you think I thought the Helios or the SR-71 can enter an orbit? I wrote "would have to", not "has to". I'm asking on the altitude at which it would have to reach orbital velocity, something the Helios isn't capable of.

@Betternottell Neither is the X-2. It was only used as an example to illustrate.

That's right, I'm doing the same here so don't claim I would think they can reach orbit.

@Koyovis it may be that Helios max altitude may have been limited by it Power available, as the wing may have been able to cruise a little higher while staying under Mach.  Hope they build another one.

The limitations of winged aircraft are far below the Karman line due to far lower air density and wastefulness of carrying the weight of a wing to those altitudes, in addition to practical sustained propulsion issues.

Lift = Density × Area × Lift coefficient × Velocity$^2$

Holding Area and Lift coefficient constant (to best Angle of Attack), a rough comparison of density to velocity$^2$ to generate Lift = Weight for various altitudes becomes possible.

Velocity$^2$ = k/Density

Since the IAS of Helios is known, let's plug in some density and velocity numbers, starting with sea level density.
Sea level:      23 knots$^2$ (TAS)    = 12574/23.77 (IAS = TAS)
10000 feet:     27 knots$^2$          = 12574/17.56
50000 feet:     59 knots$^2$          = 12574/3.64
150000 feet:   583 knots$^2$          = 12574/0.037
250000 feet:  4398 knots$^2$          = 12574/0.00065
Although it might be tempting to think at a very high altitude one might need only a little lift, the thought goes by the wayside when one realizes most of thrust in very high speed flight is involved with overcoming drag. At 4000+ knots, as the X-15 did, while still  in the atmosphere, a great amount of heat is produced, much like a meteor. Better to get above this and follow a suborbital ballistic path.
Amazingly, above most of the atmosphere, all objects will have a very similar Karman line altitude, because at those altitudes aerodynamic forces are very small, and speeds required to produce them approach orbital velocity (in theory). However, the exact point where "space" begins is debatable, as satellites in higher orbit still experience significant atmospheric drag$^1$.
To get anywheres close to the Karman line, the aircraft must overcome the sonic and thermal "barriers" if it hopes to survive for long.  Helios did neither, but did a fine job with a subsonic  True airspeed.
$^1$ see Karman reference under Alternatives to the FAI definition

What do you mean "above this"? If you're high enough / far enough from Earth air is too thin to cause heating. A suborbital ballistic path is any parabola whose perigee is within the Earth. Where do you have it from that the Helios' true airspeed is 400 kts; wouldn't Wikipedia mention that? You're above most of the atmosphere when you're above 18,000 ft (5.5 km) MSL btw but you actually mean mesosphere / lower thermosphere altitudes.

@Betternottell well, you're right, it need not be sub-orbital, it could be orbital, or going to Mars.  But the notion aircraft had any business doing this does not match the physics of lifting flight.  The Karman line is theoretical, extrapolated from a few (very dangerous) X-2 flights that didn't even come close in the 1950s.  At the altitude it did fly, it was barely controllable.  The X-15 did reach 100 km, but required rocket thrusters for control.  Helios TAS is easily derived using TAS calculator on the net.  Google it.

I know, but why do you think Wikipedia would mention IAS at record altitude only? If it mentions the maximum airspeed, then it would include sea level speeds probably, where IAS is the same as TAS. I mean the Helios ground-launched (near sea level), not mid-air.

@Betternottell it is somewhat dramatic to highlight its IAS, especially since NASA was into many wing mounted solar electric props (which greatly helped the sustained propulsion issue at those altitudes).  Really an amazing aircraft (yet another lost flown on a windy day!).


text:
Did the Concorde have darkened windows?
Did the Concorde and the Tu-144 have darker window glass than subsonic airliners, to protect from the Sun when flying above 9/10 of the atmosphere? Or were the crew and passengers at higher risk of eye damage if looking towards the Sun from cruise altitude?

@MichaelHall Thank you. I would upvote your comment now if I had the rights.

Concorde had tinted nose visor glass to withstand the heat and pressure produced at the cruise speed it could achieve. It was a laminate with a gold layer in the middle, thus the tint. I don't believe the passenger cabin glass was tinted.

Passenger windows were so small that passengers probably couldn't normally see the sun except when quite low in sky?

@quietflyer Would it matter if they see the entire solar disk or a part of it? Isn't it almost equally dangerous? One shouldn't look into the Sun during an annular eclipse either.

@Betternottell -- I wasn't implying that the disk would be partially hidden by the horizon, just noting that what little atmosphere is there has more of a darkening effect when the sun is quite low in the sky.  Actually I think this whole question could be grounds for a much more general question about whether viewing the sun is significantly more harmful when flying at high altitude than otherwise, and what if any measures have typically been taking to deal w/ this, other than telling crew/ passengers "don't look at the sun".  Well, actually, even including that one.

@quietflyer The higher the altitude the brighter the Sun is and the less filtered UV radiation is by the thinner atmosphere, but if I understood you correctly you mean that in Concorde windows the Sun would only be visible low enough so when looking through enough atmosphere it wouldn't be more dangerous than on the surface.

It should be noted that glass is reasonably effective at blocking UVB and UVC radiation, which cause sunburn and might reasonably be blamed for permanent damage to the eyes (visible light can cause bleaching of the optical receptors but this is reversible).